1. Field of the Invention
The present invention relates to a rotor blade for a rotary-wing aircraft and, more particularly, to an airfoil section of a rotor blade for a rotary-wing aircraft, having a small drag coefficient in a state where the Mach number is small.
2. Description of the Related Art
Airfoil sections developed by NACA, i.e., the former NASA (National Aeronautics and Space Administration) of USA, are used widely. Airfoil sections NACA0012 and NACA23012 are representative ones among those developed by NACA
The applicant of the present patent application proposed the airfoil sections with improved performance in the U.S. Pat. No. 5,344,102. Each of previously proposed airfoil sections has a maximum lift coefficient C.sub.1max and a zero-lift drag divergence Mach number M.sub.dd, which are higher than those of the NACA's airfoil sections. Shown in FIGS. 11(A), 11(B), 11(C), 11(D) and 11(E) are airfoil sections U896H-10, U896H-10UR, U896H-08, U896H-09 and U896H-12 proposed in the U.S. Pat. No. 5,344,102. Since the upper line and the lower line of each of these airfoil sections are substantially symmetrical in a range between coordinates corresponding to about 30% chord length and coordinates corresponding to about 90% chord length, the nose-down pitching moment about the airfoil section, with intrinsic characteristics to cambered airfoil sections, is comparatively small. Accordingly, these airfoil sections can reduce the load on the pitch angle changing mechanism of a rotary-wing aircraft and vibrations due to the variation of the pitching moment during the rotation of the rotary wing.
FIG. 12 shows the performance of NACA0012, NACA23012 and the airfoil sections proposed in the U.S. Pat. No. 5,344,102, in which the maximum lift coefficient C.sub.1max at a Mach number of 0.4 is indicated at a higher point along the vertical axis and the zero-lift drag divergence Mach number M.sub.dd is measured to the right position along the horizontal axis. As is obvious from FIG. 12, the maximum lift coefficients C.sub.1max and the zero-lift drag divergence Mach numbers M.sub.dd of the airfoil sections of the U.S. Pat. No. 5,344,102 are larger than those of NACA0012 and NACA23012, which indicates the excellent performance of the airfoil sections proposed in the U.S. Pat. No. 5,344,102.
An inner wing portion, i.e., the base portion, of the rotor blade for the rotary-wing aircraft and an outer wing portion, i.e., an wing tip portion, of the same undergo different airflow conditions, respectively, during operation. Therefore, it is inappropriate, in some cases, to apply the same airfoil sections previously proposed by the applicant of the present patent application to all the sections of rotor blades. The airspeed of the inner part of a rotor blade is lower than that of the outer part and the working Mach number of the inner part is comparatively small. Therefore, requirements of the inner part of a rotor blade concerning drag divergence Mach number are not very severe. Generally, the maximum lift coefficient C.sub.1max of an airfoil section is comparatively large in a small Mach number range, the maximum lift coefficient C.sub.1max of the inner part need not be very large. Therefore, the improvement of the drag characteristic of the inner part of a rotor blade under a hovering flight condition, where Mach number is about 0.6 and lift coefficient is about 0.6, is more important than increasing the maximum lift coefficient and the drag divergence Mach number, and the reduction of the drag coefficient under such airflow conditions is required.
Although the airfoil sections proposed in the U.S. Pat. No. 5,344,102 each have a large maximum lift coefficient C.sub.1max or a large zero-lift drag divergence Mach number M.sub.dd, the drag coefficient shows a tendency to increase when the lift coefficient is 0.6 or above and the Mach number is about 0.6, and the airfoil section of the inner wing portion of the rotor blade leaves room for improvement.